1.Technical Field
This disclosure generally relates to synchronization signal generators and, more particularly, to a synchronization signal generator, which generates a pixel clock for switching an image signal supplied to an optical beam modulator at delimitation of pixels, and an image forming apparatus using such a synchronization signal generator.
2. Description of the Related Art
There are known various methods for an image-forming process used in an image forming apparatus, which forms a color image according to electrophocography. Among those methods, there is a method that is referred to as a tandem type. In this method, a photo conductor and an image-forming process element are provided for each color component of a color image to be formed. The photo conductor and the image-forming process element are arranged along an intermediate transfer member and a paper conveyance belt. Images formed in each color component are superimposed on the intermediate transfer member, and the superimposed full-color image is transferred onto a recording paper at once. Alternatively, a color image formed on each photo conductor is transferred onto a recording paper each time the recording paper, which is conveyed by the paper conveyance belt, passes through the transfer process part of each photo conductor so that a full color image is formed by causing the recording paper to pass through all transfer stations.
FIG. 1 shows a structure of a tandem type color image forming apparatus. In FIG. 1, photoconductor drums 6a-6d, which form images in different colors (yellow: Y/y, magenta: M/m, cyan: C/c, black: K/bk), are arranged in a single row along a conveyance belt 10, which conveys a transfer paper (recording paper). According to an image signal for recording, laser beams modulated by image signals for Y, M, C and K recording are projected and scanned on the respective photoconductor drums 6a, 6b, 6c and 6d that have been charged uniformly by an electric charger so as to form electrostatic latent images thereon. Each electrostatic latent image is developed by respective Y, M, C, and K toner in respective developer 7a, 7b, 7c and 7d so as to Form toner images (developed images: visible images) in each color.
The transfer paper is conveyed onto the transfer belt 10 of the transfer belt unit from a paper feed cassette 8. Color images developed on the photoconductor drums are sequentially transferred onto the transfer paper by transfer units 11a, 11b, 11c and 11d in a superimposed state, and are fixed by a fixing device 12. The transfer paper after completion of the transfer is ejected out of the image forming apparatus.
The transfer belt 10 is a transparent endless belt supported by a drive roller 9, a tension roller 13a and an idle roller 13b. Since the tension roller 13a presses down the belt 10 with a spring (not shown in the figure), a tension applied to the belt 10 is substantially constant.
FIG. 2 is a top plan view of an optical unit constituting an exposure unit 5. In FIG. 2, light beams from laser diode units (LD units) 31bk and 31y (each including a laser diode and a laser driver that modulates a laser beam) pass through respective cylinder lenses 32bk and 32y, and are deflected by respective reflection mirrors 33bk and 33y and incident on a surface of a lower side of a polygon mirror 34. The light beams are deflected by the rotating polygon mirror 34, pass through fθ lenses 35bkc and 35ym and are folded by first mirrors 36bk and 36y, respectively.
On the other hand, light beam from laser diode units 31c and 31m pass through respective cylinder lenses 32c and 32m, and are deflected by respective reflection mirrors 33c and 33m and incident on a surface of an upper side of the polygon mirror 34. The light beams are deflected by the rotating polygon mirror 34, pass ,through fθ lenses 35bkc and 35ym and are folded by first mirrors 36c and 36m, respectively.
Cylinder mirrors 37bkc and 37ym and sensors 38bkc and 38ym are provided on an upstream side of a write-start position in the main scanning direction. The light beams passed through the fθ lenses 35bkc and 30ym are reflected and converged by cylinder mirrors 37bkc and 37ym and are incident on the sensors 38bkc and 38ym, respectively. The sensors 38bkc and 38ym serve as synchronization detection sensors for acquiring synchronization in the main scanning direction.
Additionally, cylinder mirrors 39bkc and 39ym and also sensors 40bkc and 40ym are provided on a downstream side of an image area in the main scanning direction. The light beams passed through the fθ lenses 35bkc and 35ym are reflected and converged by cylinder mirrors 39bkc and 39ym and incident on the sensors 40bkc and 40ym, respectively.
Moreover, in the detection of the light beams from the LD units 31bk and 31c, the common sensor 38bkc is used on the write-start side and the common sensor 40bkc is used on the write-end side. Similarly, in the detection of the light beams from the LD units 31y and 31m, the common sensor 38ym is used on the write-start side and the common sensor 40ym is used on the write-end side. Since the light beams for two color images are incident on the same sensor, the incident angles of the light beams of each color are set different from each other so as to vary timings of the light beams entering each sensor, thereby outputting the light beams in a pulse train. As interpreted from the figure, the light beams of K(bk) and C(c) and the light beams Y(y) and M(m) are scanned in opposite directions.
FIG. 3 is a block diagram of a conventional synchronization signal generator. The synchronization signal generator shown in FIG. 3 has PLL (Phase Locked Loop) units 40bk, 40M, 40C and 40Y for each color. In the PLL units 40bk, 40M, 40C and 40Y, a reference clock REFCLK is M-divided by dividers 43bk, 43M, 43C and 43Y (divided to 1/M frequency), and a high-frequency clock *_PLLCLK of the same frequency is N-divided by dividers 45bk, 45M, 45C and 45Y. Signals output from the dividers are supplied to PLL control units 44bk, 44M, 44C and 44Y, each of which comprises a phase comparator, a low-pass filter (LPF) and a voltage-controlled frequency variable oscillator (Vco), so as to generate a high-frequency clock *_PLLCLK for each color. It should be noted that, specifically, the high-frequency clock *_PLLCLK contains K_PLLCLK, M_PLLCLK, C_PLLCLK and Y_PLLCLK. The high-frequency clock *_PLLCLK is K-divided by dividers 41bk, 41M, 41C and 41Y in accordance with a synchronization detection signal *_DETP_N from the above-mentioned synchronization detection sensors 38bkc and 38ym so as to generate pixel clock *_WCLK for each color. The synchronization detection signal *_DETEP_N and the pixel clock *_WCLK are supplied to internal synchronization signal generation units 42bk, 42M, 42C and 42Y so as to generate internal synchronization signal *_PSYNC_N (line synchronization signal) which synchronizes with the pixel clock *_WCLK.
Specifically, the pixel clock *_WCLK includes K_WCLK, M_WCLK, C_WCLK and Y_WCLK. Hereinafter, the pixel clock *_WCLK may be simply referred to as WCLK, and the same may be applied to other signals.
The image forming section of each color controls the output timing of the image signals (switching of image signals) used for LD modulation in the LD units 31bk, 31m, 31c and 31y by using the pixel clock *_WCLK and the internal-synchronization signal (line synchronization signal) *_PSYNC_N.
The frequency fwclk of the pixel clock *_WCLK is represented as fwclk=frefclk/M×N/K. In this expression, frefclk is the frequency of a reference clock REFCLK. Japanese Laid-Open Patent Application No. 2000-221431 discloses a pixel clock generator, which uses a PLL unit for each color.
When performing a magnification correction for main scanning, that is, when performing a frequency correction of the pixel clock *_WCLK so as to equalize a number of pixels on each color line, the pixel clock *_WCLK is counted after the front-end (start) detection sensors 38bkc and 38ym detect the light beams and until the rear-end (end) detection sensors 39bkc and 39ym detect the light beams. Then, the values of M and N are adjusted so that the count value becomes equal to a predetermined reference count value. That is, the frequency of the pixel clock *_WCLK is adjusted so as to equalize the number of pixels on each color line. Japanese Lad-Open Patent Application No. 2002-096502 discloses an exposure apparatus which performs the above mentioned magnification correction of main scanning.
It is possible to adjust a magnification (number of pixels) of whole one line by the magnification correction, which is for changing the frequency of the pixel clock. However, it is difficult to correct a local expansion and contraction (positional offset of pixels in the main scanning direction) due to positional inaccuracy of the optical lenses (fθ lens) and mirrors in the optical system. Japanese Patent No. 3231610 discloses a method of correcting an expansion and contraction in a width of an image in the main scanning direction. In this method, three kinds of clocks having a reference period, a period shorter than the reference period and a period longer than the reference period are prepared so as to increase and decrease the width of pixel (pixel-width) by selectively setting one of the clocks in accordance with a main scanning position (position of a pixel in the main scanning direction).
If the PLL unit is provided for each color, provision of many PLL units may increase a cost. If both the magnification correction in the main scanning direction and the correction of pixel-width in the main scanning direction are performed simultaneously, and when data for magnification correction in the main scanning direction and data for correction of expansion and contraction of pixel-width (variation in the pixel-width) are synthesized by superimposing them by developing on a memory as disclosed in the above mentioned Japanese Patent No. 3231610, a large capacity of memory for one line is needed which results in a cost increase. In a case where the magnification correction in the main scanning direction is performed between pages, if the set value of the PLL is changed, a considerable time period is needed until the PLL becomes stable, which decreases productivity in continuous printing.